2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-02-1
VALIDATION OF THE GEANT4 CODE IN THE EVALUATION OF ORGAN DOSE EQUIVALENTS IN A MIRD-5 PHANTOM Rosana de Souza e Silva1, Márcia Begalli1, Pedro Pacheco de Queiroz Filho2 and Denison de Souza Santos2 1
Departamento de Física de Altas Energias Universidade do Estado do Rio de Janeiro Rua São Francisco Xavier, 524. 20550-900 Rio de Janeiro, RJ
[email protected],
[email protected] 2
Instituto de Radioproteção e Dosimetria (IRD / CNEN - RJ) Av. Salvador Allende, s/n 22780-160 Rio de Janeiro, RJ
[email protected],
[email protected]
ABSTRACT The evaluation of dose of ionizing radiation received by an individual or by particular organs may be obtained by the numerical simulation of particle transport through mathematical models that represent the human body. Among these models are those developed by the Medical Internal Radiation Dose Committee, MIRD, known as “MIRD-5 Phantoms” or “Snyder-Fisher Phantoms”. Such phantoms, which represent human body organs by geometrical solids, are widely used in medicine applications such as dose planning in therapeutic or diagnostic treatment. The simulation techniques used, known as Monte Carlo methods, rely on codes that have been extensively validated such as MCNP and EGSnrc. Geant4, a code developed at CERN by the high-energy physics community, has been recently used in radiation protection applications, where radiation energies are usually much inferior to those found in particle accelerators. Geant4 is an open source code, written in C++ for Unix, Linux or Windows platforms. The validation of Geant4 in the context of low energies is necessary once the code has already been largely validated in the applications for which it was initially designed. In this work we build a MIRD-5 type phantom using objects available to Geant4, obtain radiation doses to its organs when irradiated in a photon parallel plane field and validate the results against those obtained by other accepted methods. Visualization tools (Open Inventor) and the statistical analysis tool (AIDA) are also open source code, gathered in the OpenScientist package.
1. INTRODUCTION Radiation Protection standard values for dose calculation adopted by the ICRP [1]have been calculated using the so-called MIRD-5 [2] type human phantoms which simulate the human body by using a series of surfaces to describe all of its organs. Those phantoms receive this name after the Medical Internal Radiation Committee that introduced them in its pamphlet number 5. Initially describing a hermaphrodite phantom, those models evolved into separate male and female phantoms and, later, to several descriptions of humans as children of various ages and pregnant women [3] and have been revised in MIRD Pamphlet 15 [4]. All of those models have been developed for use in Monte Carlo calculations using several different codes like EGSnrc [5]and MCNP [6].
In the present work we implement a MIRD-5 type phantom for use with the Geant4 code. Geant4 [7, 8, 9] is a Monte Carlo simulation code developed at the European Organization for Nuclear Research (CERN) [10], with the purpose of describing particle physics experiments, where detectors may contain hundreds of sub-elements and particles may reach energies as high as hundreds of GeV. The code is written in C++, runs on UNIX, LINUX or WINDOWS® platforms and is publicly available for download from its home page. The physics described by this code has been extensively validated for those energies but, for purposes of radiation protection, it is still necessary to show the validity of the description when energies are of the order of a few keV. The human phantom implemented here was introduced by Kramer et al. and describes a male adult mathematical phantom known as ADAM [11]. Monte Carlo calculations of organ doses for this phantom have been published by Zankl et al. [12] and were used here in the validation of our results. In the construction of our phantom, a total of ?? volumes were defined to emulate ADAM. Whereas in the original description, organs were described by quadratic inequalities, Geant4 already defines a series of classes to represent volumes as spheres, cylinders, ellipsoids, etc. that fit in the representation of organs. In addition to those, the code allows for the sum or subtraction of geometrical shapes when defining a logical volume. Such a feature was used in the implementation of more complex geometries as the thyroid, for example.
2. MATERIALS AND METHODS
The simulation code used was Geant4, version 4.8.p01 and its cross section data, G4EMLOW4.2. Statistical analysis and histogram creation was done by means of the OpenScientist 16.0 [13] implementation of AIDA [14]. The visualization package was OpenInventor 2.1.5-10 [15]. The random number generator chosen was RANMAR. For each run, 1.0 x 106 photons were generated on a Pentium IV, 2.4 GHz processor, with 512 MB SDram memory running a LINUX OS. The geometry of irradiation was always a parallel plane of photons. Following the reference paper, the electrons cut chosen was 200 keV.
3. RESULTS AND DISCUSSION
Figure 1 shows a front view of the ADAM phantom. Visualization is performed by the OpenInventor package, used by Geant4, but developed independently. This package also allows the visualization of trajectories and interaction points during execution time as well as a three dimensional rotation of the point of view and zooming in and out. It is one of four available packages for visualization in Geant4.
INAC 2007, Santos, SP, Brazil.
Figure 1. Front view of the ADAM phantom implemented in Geant4.
For the different energies used, Table 1 shows the ratio of equivalent organ doses calculated by Geant4 and those quoted in reference [12]. Figure 2 shows results obtained in a plane parallel irradiation of 50 keV photons, compared to those of the reference [12].
Table 1. Ratio of equivalent organ doses calculated by Geant4 and by Zankl et al. Orgãos Testes Red bone marrow Skeleton Lungs Stomach Bladder Liver Oesophagus Thyroid Ascending Colon Descending Colon Skin Adrenals Brain Small intestine Kidney Pancreas Spleen Thymus Eyes
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50 keV 0,91 1,26 0,98 0,66 0,89 0,92 1,26 0,84 0,72 0,93 0,93 1,29 0,86 1,09 1,11 0,99 0,99 0,99 0,87 1,08
100 keV 1,05 1,06 0,98 0,88 0,94 0,92 1,26 0,85 0,82 0,95 0,90 1,31 1,03 0,99 1,09 0,97 0,96 0,94 0,89 0,96
200 keV 0,95 0,99 0,99 0,98 0,95 0,99 1,29 0,84 0,83 0,97 0,91 1,29 1,07 0,98 1,11 0,95 1,05 0,97 0,89 0,98
300 keV 1,03 0,91 1,00 0,99 0,99 0,98 1,30 0,82 0,87 0,94 0,91 1,29 1,02 0,99 1,11 0,98 1,04 1,00 0,97 1,16
500 keV 1,07 0,88 1,00 1,00 0,98 0,97 1,31 0,79 0,87 0,98 0,95 1,28 1,07 0,98 1,12 0,95 1,09 1,01 1,07 0,92
1000 keV 0,99 0,85 0,99 1,04 1,02 0,89 1,31 0,81 1,07 0,98 0,94 1,16 1,35 1,00 1,13 0,99 1,03 1,03 0,98 0,95
Figure 2. ADAM equivalent organ doses per air kerma in plane parallel irradiation of 50 keV photons. Results obtained by Geant4 and by Zankl et al. (reference values).
Ratio Geant4/Reference
ADAM - AP Irradiation 1,40 1,20
Testes
1,00 0,80
Red bone marrow Skeleton
0,60
Skin
0,40
Eyes
0,20 0,00 0
200
400
600
800
1000
Energies (keV)
Figure 3. Ratio of Geant4 organ equivalent doses to reference [12] as a function of energy, for five organs.
Organ equivalent doses calculated by Geant4 show no general tendency to overestimate or to underestimate the values taken as reference. As the energy increases, there is no general bias INAC 2007, Santos, SP, Brazil.
neither, as some of the organ ratios increase and others decrease. A difference of up to 30 percent was found in either direction from the reference values.
4. CONCLUSIONS It is possible to use the Geant4 simulation toolkit to implement a MIRD-5 type human phantom. The calculation of organ equivalent doses is consistent with previously published results. The importance of using this kind of Monte Carlo code to evaluate organ doses is that by the very nature of its design, it is suitable for use with several types of particles for the incident beam, like neutrons, protons, positrons, pions, etc and also for energies as high as GeV. This may be of great help in radiation therapy applications with new hadron accelerator machines and in unusual situations as cosmic rays irradiations in spatial flights. From this point a family of phantoms may also be implemented with different irradiation geometries. The implementation of voxel phantoms is also foreseen.
ACKNOWLEDGMENTS The authors wish to express their gratitude to Dr. Richard Kramer and his student Vagner Cassola for guidance on the calculation of red bone marrow doses. This work was partially financed by CNPq.
REFERENCES 1. http://www.icrp.org/, accessed on June, 17th, 2007; 2. Snyder, W. S., Ford, M. R., Warner, G. G., Fisher, H. L.Estimates of Absorbed Fractions for Monoenergetic Photon Sources Uniformly Distributed in Various Organs of a Heterogeneous Phantom - MIRD Pamphlet No. 5. J. Nucl. Med, 10(3) (1969). 3. Cristy’s paper on children phantoms. 4. L. G. Bouchet et al., “MIRD Pamphlet No 15: Radionuclide S Values in a Revised Dosimetric Model of the Adult Head and Brain”, J. Nucl. Med. 40, pp 62S-101S (1999); 5. http://www.irs.inms.nrc.ca/EGSnrc/EGSnrc.html, accessed on June, 17th, 2007; 6. http://mcnp-green.lanl.gov/index.html, accessed on June, 17th, 2007; 7. http://Geant4.web.cern.ch/Geant4/, accessed on June, 17th, 2007; 8. S. Agostinelli et al., “Geant4 - A Simulation Toolkit”, Nuclear Instruments and Methods A 506 pp 250-303 (2003); 9. J. Allison et al., “Geant4 Developments and Applications”, IEEE Transactions on Nuclear Science 53 No. 1, pp 270-278 (2006); 10. http://public.web.cern.ch/Public/Welcome.html, accessed on June, 17th, 2007; 11. Kramer, R., Zankl, M., Williams, G. and Drexler,G. The Calculation of dose from External Photon Exposures Using Reference human Phantoms and Monte Carlo Methods-Part I: The male (Adam) and Female (Eva) Adult Mathematical Phantoms. Institute für Strahlenschutz, GSF-Bericht S-885 (1982). 12. Zankl, M., Petoussi-Henβ, N., Drexler, G., Saito, K., The Calculation of Dose from External Photon Exposures Using Reference Human Phantoms and Monte Carlo Methods – Part VII: Organ Doses due to Parallel and Environmental Exposure Geometries. Institute für Strahlenschutz, GSF-Bericht 8/97 (March/1997);
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13. http://openscientist.lal.in2p3.fr, accessed on June, 17th, 2007; 14. http://aida.freehep.org, accessed on June, 17th, 2007; 15. http://oss.sgi.com/projects/inventor, accessed on June, 17th, 2007
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